1. Field of the Invention
This invention relates to engine control systems using ionization probes as sensing elements.
2. Prior Art
It is known to control various engine operating parameters, such as composition of exhaust emissions, in accordance with the relative time of occurrence of the combustion chamber peak pressure and piston top dead center during the combustion process. One of the known methods of detecting combustion chamber pressure is using an ionization probe placed in the combustion chamber.
For example, U.S. Pat. No. 4,304,203 issued to Garcea et al; U.S. Pat. No. 4,377,140 issued to Latsch and British Pat. Nos. 1,512,213 and 2,060,062A teach the use of an ionization sensing probe to control the combustion cycle in an internal combustion engine. Closed loop control using an ionic current sensor to determine the end of the ignition phase of a combustible mixture in an internal combustion engine can be used to adapt ignition timing to compensate for conditions such as thermal status of the engine, characteristics of the fuel and of the combustion air, engine wear and so on.
U.S. Pat. No. 4,345,154 issued to Bainbridge teaches an ionization sensing device to detect harmful gases in a gaseous medium. To compensate for erratic changes in the electrical output signals of the sensing cell caused by variations in the flow of gas, a bias voltage, requiring a separate bias battery, is impressed upon the sensing device.
The ionization probe usually includes a metallic probe that is inserted through the cylinder wall and is electrically insulated from the metal cylinder block. When combustion occurs in the cylinder, a number of free ions are created in the flame front. The free ions in the flame front cause a current to flow and result in a decrease in the effective resistance between the probe and the cylinder walls. Typical resistance changes are from open circuit, indicating no combustion, to one to two megohms during combustion. The probe is used to measure time of arrival of the flame front relative to the spark event. The time of arrival data is used to determine the quality of the burn cycle.
Thus, the resistance of the ionization probe changes as a function of the occurrence of combustion and the application of a bias voltage to the ionization probe causes a change in the ionization probe current upon detection of combustion. Typically, applying a bias voltage to the ionization probe requires a high voltage power supply in addition to the vehicle battery. It would be desirable to eliminate this requirement. These are some of the problems this invention overcomes.